Mass fish death reported in Kotka, Finland due to hydropower plant discharge issues

New details have emerged regarding a mass fish death in Kotka, Finland. At the beginning of September, hundreds of salmon fry were found dead along the Kymijoki River at Langinkoski. The small fish were located on dry land, in side channels, and in puddles, with the death toll being unusually high.

The deaths were noticed after the Koivukoski hydropower plant, located at the river’s upper end, reduced the water flow in the river. According to regulatory conditions, the discharge should not drop below 20 cubic meters per second. However, it has now been revealed that the actual flow was significantly lower, at just 15 cubic meters per second—five cubic meters less than permitted.

This issue was discovered during research conducted by Kolsin Voima, which operates the Koivukoski plant. Kolsin Voima attributed the reduced discharge to a technical malfunction. They noted that on September 1, they had made a planned flow reduction to 20 cubic meters per second, but the water level dropped significantly afterward. Upon investigation, it was found that the transmitter measuring the position of the discharge gate had malfunctioned, leading to incorrect readings that were recorded in the authorities’ database.

Kolsin Voima has commissioned an assessment from Kymin Kalastus, an aquatic expert, regarding the fry mortality in the river’s side channels. According to this assessment, a flow rate below 20 cubic meters per second had a relatively minor impact on additional salmon fry mortality.

Repair work on the malfunctioning transmitter will commence as soon as the necessary spare parts are received, and until then, the gate will be managed manually. Further investigations will continue in collaboration with regional water and fisheries authorities to determine necessary follow-up actions.